Group III nitride compound semiconductors have been used for producing LEDs and LDs, because Group III nitride compound semiconductors have a band gap of direct transition type having energy corresponding to a domain ranging from visible light to ultraviolet rays and can efficiently emit light.
In addition, Group III nitride compound semiconductors have potential for use in an electronic devices, providing characteristics which cannot be obtained using conventional Group III to Group V compound semiconductors.
The following documents are considered:
[Patent Document 1]
The Japanese Patent Publication No. 3026087
[Patent Document 2]
Japanese Patent Laid-Open No. H04-297023
[Patent Document 3]
Japanese Patent Publication No. H05-86646
[Patent Document 4]
The Japanese Patent Publication No. 3440873
[Patent Document 5]
The Japanese Patent Publication No. 3700492
[Non-patent Document 1]
The 21st century alliance symposium memoirs, Vol. 2, p. 295 (2003)
[Non-patent Document 2]
Vacuum, Vol. 66, p. 233 (2002)
In general, Group III nitride compound semiconductors are produced using a metal organic chemical vapor deposition (MOCVD) method.
The MOCVD method uses trimethyl gallium, trimethyl aluminum and ammonia as raw materials, and adds the raw material in the form of vapor to a carrier gas to convey the resultant gas to the surface of a substrate, thereby decomposing the raw material through the reaction with the heated substrate to grow a crystal.
Although the MOCVD method has a merit of allowing fine control of film thickness and control of composition, it has a demerit in that the deposition takes longer and the control of the parameters is difficult.
Therefore, production of Group III nitride compound semiconductors using a sputtering method has been studied.
The sputtering method can increase the deposition rate, and allows simple and easy control of the parameters, thereby providing effects largely in the productivity of the device.
There is a report that a GaN layer can be deposited using a sputtering method to form a layer having an excellent smoothness (Non-patent Document 1 and Non-patent Document 2).
Non-patent Document 1 discloses that a GaN layer was deposited on Si(100) and sapphire (Al2O3)(0001) by a high-frequency magnetron sputtering using N2 gas.
As conditions for deposition, the total gas pressure was 2 mTorr, the applied electricity was 100 W, while the temperature of the substrate was changed from room temperature to 900° C. In accordance with the drawing shown in the monograph, the apparatus used there faced a target and a substrate to each other.
In addition, the Non-patent Document 2 discloses that a GaN layer was deposited using an apparatus in which a cathode and a taget were arranged to face to each other and in which a mesh was inserted between the substrate and the target.
As conditions for deposition, the pressure of the N2 gas was set to 0.67 Pa, the temperature of the substrate ranged from 84 to 600° C., the applied electricity was 150 W, and the distance between the substrate and the target was 80 mm.
In addition, a study has been started in which a sputtering method and an MOCVD method have been used in the production of a laminated structure of a Group III nitride compound semiconductor using GaN to control an exact thickness and a resulting composition for producing a smooth deposition quickly and with no defects, thereby improving the speed of production.
Each of Patent Documents 1 and 2 disclose incorporating a buffer layer such as AlN in laminating GaN layer on a sapphire substrate to eliminate lattice incommensurate between GaN and sapphire layer, thereby increasing crystallinity of GaN.
In addition, it has been reported that an epitaxial growth can be performed by depositing a buffer layer using a high frequency sputtering method, and then growing a crystal having the same composition on the buffer layer using the MOCVD method (Patent Document 3).
It has been reported that the characteristics of the epitaxial growth can be improved by incorporating an annealing treatment of a buffer layer in the depositing process (Patent Document 4). In addition, it has been reported that the characteristics of the epitaxial growth can be improved by deposition of a buffer layer using a DC sputtering method (Patent Document 5) at a temperature of not less than 400° C.
Thus, the importance of a technique to deposit a Group III nitride compound semiconductors using a sputtering method has increased, and in particular, the importance of a technique to deposite GaN using a sputtering method has increased. However, since Ga is a liquid at normal temperature, until now it has been necessary to cool Ga to a solid state before sputtering deposition is performed when Ga is deposited as a target.
However, when the target is insufficiently cooled, or the power of plasma to be applied is increased, Ga suddenly changes from a solid state into a liquid state, and the deposition therefore may not be controllable.
Since a conventional backing plate had been produced on the assumption that Ga was used in a solid state, the wettability with Ga in a liquid state has not been considered.
When a material having poor wettability was used in the surface which comes into contact with Ga of a backing plate, heat did not escape from Ga to the backing plate, and as a result, the heat might melt Ga, or the cooling effect of the backing plate which was cooled down by a cooling agent did not conduct to Ga successfully, and as a result, the Ga might not be cooled down.
In such a case, the state of Ga suddenly changes from a solid into a liquid, thereby making the deposition control unstable.
In addition, when Ga was in a liquid state, Ga gathered into a droplet by surface tension, thereby exposing the surface of the backing plate.
As a result, a constitution element of backing plate surface was sputtered therewith.
Actually, when a sputtering was performed when the backing plate surface was exposed, and the resultant deposited product was analyzed, it revealed that an element which constitutes the backing plate was contained therein as a contaminant.
Accordingly, it is necessary to stably deposit a layer stably by uniformly performing thermal conduction between Ga and the backing plate whether Ga is in a liquid or solid state. In addition, when Ga is in a liquid state, it is necessary to form a Ga nitride compound semiconductor having no impurities by spreading Ga in a liquid state over the sputtering surface of the backing plate so as to deposit without exposing the surface of the backing plate.